1. Field of the Invention
The present invention relates generally to the field of x-ray collimators. More specifically, the present invention discloses an x-ray collimator for use primarily in x-ray proximity lithography for semiconductor fabrication.
2. Statement of the Problem
X-ray lithography has been used experimentally in the past for etching semiconductor wafers. However, existing x-ray lithography systems have not been commercially viable due to a number of significant shortcomings, particularly the speed, cost, complexity, and size of such x-ray lithography systems.
For success in a commercial environment, an x-ray lithography system should be able to meet stringent collimation requirements. Specific performance requirements have been set by Sematech, as part of a roadmap for the future of lithography. Until now, no x-ray collimator has been able to achieve all of the requirements simultaneously. In particular, the tight beam uniformity requirement had never been solved. The x-ray collimator must reduce global divergence to remove all pattern shadowing in the resist, i.e., approximately 20 milli-radians over a 30 mm square mask. Local beam divergence should be reduced to below the level of diffraction in the mask features, i.e., approximately 10 milli-radians diameter. Beam uniformity should be within .+-.3% everywhere across the 30 mm square wafer.
In the exposure of an x-ray photoresist, as with a visible light photoresist, a variation in beam intensity will cause a variation in the exact width of the resultant feature. This effect is called depth of exposure. High precision circuits have strenuous limitations on feature width variations, so a tight constraint on exposure uniformity and beam uniformity is required. The .+-.3% beam uniformity specification is very hard to meet in the x-ray realm, where optics are highly absorbing and non-uniform.
The x-ray collimator must also meet gain requirements assuming a 100 W/steradian pinch source to deliver a beam with sufficient flux to support a production rate of 25 to 50 wafers per hour. In addition, the collimator optics must be robust and reliable. The x-ray collimator should not significantly increase the overall cost of the stepper, and should not have excessive space requirements.
The gain requirement is driven by the need for adequately fast systems. If speed were not the driving consideration, then the source could simply be moved back to four or five meters from the mask and meet the divergence criterion. The purpose of the collimator is to provide the low divergence with adequate signal. The speed of the stepper is controllrd by the wafer handling time, the step and align time per die, and the exposure time per die. A conventional stepper uses 22 seconds to insert and remove a wafer. It also requires one second per die to step and align. Assuming a typical number of 20 dice per wafer, we can then write an expression for the number of wafers per hour achievable: ##EQU1## where W is the wafers per hour, a is the step and align time in seconds, and e is the exposure time in seconds. Since a is known to be 1 sec, with an exposure time of zero, the system would handle 85 wafers per hour. In Table 1, we show the expected system throughput as a function of the exposure time:
TABLE 1 ______________________________________ System Speed Exposure (sec) Wafers/hr Beam (mW) ______________________________________ 0 85 .infin. .25 76 1800 1 58 450 1.5 50 300 2 43 225 3 35 150 5 25 90 8 17 56 10 14 45 15 10 30 ______________________________________
To achieve 10 wafers per hour requires 15 second exposures, 20 wafers per hour requires 7 second exposures, and 30 wafers per hour requires 4 second exposures. To convert exposure time to beam intensity we must assume a resist sensitivity. As sensitivity is a function of wavelength, we must choose a number that is representative of the speed after convolution with the incident spectrum. For our purposes, an exposure of 50 mJ/cm.sup.2 is reasonable. It will thus require 450 mJ to expose a 30 mm square. These numbers have been used to generate the third column of Table 1.
Inspection of the above table gives some sense of the beam requirements for an x-ray collimator. First, there is no sense in pushing much above one watt in the collimated beam, because the exposure time has already dropped to a negligible fraction of the time spent on each die. Similarly, a beam with less than about 20 mW will yield below 10 wafers an hour and render the stepper commercially non-viable unless it is quite inexpensive. The present collimator has been designed around these numbers.
3. Solution to the Problem
None of the prior art shows a collimator for use in x-ray lithography that meets all of the conditions for commercially viable in terms of speed, cost, complexity, size, throughput, and collimation tolerances for an x-ray lithography systems. In particular, none of the prior art systems have been able to combine high beam intensity with the high beam uniformity required by depth of exposure considerations. In contrast, the present system is able to meet these stringent requirements using multiple sets of small, flat or curved mirrors at grazing incidence, bundled tightly and placed at the correct distance from the mask and wafer.